1. Field of Invention
My invention relates to the field of optics, and more particularly to real time processing of optical signals with incoherent light.
2. Brief Description of the Prior Art
Real time optical signal processing technology began to develop at least as early as 1946, with processors capable of forming Hilbert transform pairs and limited voice processing. Processors using accoustooptic modulators date back to at least 1948, as illustrated by U.S. Pat. No. 2,451,465 to Barney. Since that time, interest in optical signal processing has steadily increased, despite the appearance and maturing of digital technology.
Though digital processors offer flexibility presently not possible in optics, interest continues to be drawn to optical solutions for a variety of reasons. The foremost of these is input bandwidth. While digital operation is difficult above bandwidths of a few tens of megahertz and nearly impossible above one hundred megahertz, acoustooptic processors routinely process bandwidths from the low megahertz region to a gigahertz. Optical signal processors operate exceptionally fast, with outputs available milliseconds to less than a microsecond after the input process is complete. The parallelism achieved in optics is also advantageous. It is not uncommon to perform operations on more than 10.sup.5 data elements, and with no more components than it takes to operate on a few. Still another feature of optics is that many desirable operations naturally lend themselves to easy implementation. The most important of these is the Fourier transform, which occurs in an almost trivial way in the optical domain. Multiplication is performed trivially, as well. Since optical systems usually have few components, they are often superior to their digital counterparts in size, power consumption, cost, and reliability.
Optical processors do not presently lend themselves to general problem solving by arbitrary programming. Instead, they are special purpose computers designed for specific tasks, capable of only very limited programming. As a result, a major portion of optical signal processing research is devoted to devising architectures to accomplish each desired operation. The operations performed by optical processors generally take the form of integral transforms in which the integrals are formed by one or both of two methods. Processors which accumulate the integral by collecting light over a period of time are termed "time-integrating processors." Optical architectures which achieve integration by virtue of the Fourier transform property of the lens are called "space integrating processors."
It was previously believed that most space-integrating processors--those which use the whole Fourier transform offered by a lens--required coherent light. The prototypical acoustooptic processors in this category are: the optical "excisor"--a programmable filter of electrical signals; the multichannel radiometer--a device to estimate the power spectrum of a signal accompanied by noise; and the ambiguity processor--a time versus frequency cross-correlator.
Techniques and devices are known in the prior art which allow these processors to be constructed, but only if coherent light is used as the illuminating source. This restriction is an undesirable limitation because, with incoherent light capability: light-emitting diodes can be used for sources; coherent "artifacts" are avoided; precisely controlled illumination is not required, in the sense that very poor collimation may be tolerated; and in some cases, the necessary optical system equipment may be simplified.
A fundamental problem with implementing an incoherent light optical processor lies in the difficulty in performing Fourier transforms with incoherent light. The possibility of performing this function has been predicted theoretically, and systems have been demonstrated which calculate the transform on a static signal such as a transparency. There is no known way to perform an equivalent function on a dynamic signal, nor is such a step straightforward, for real time processing and the use of real time modulators raise analytical questions not present in the static formulation. It would be desirable if incoherent light processors were available.